Stereochemically Active Lone Pair Effect of Cations Triggers Giant Rashba–Dresselhaus Spin Splitting in Ferroelectric Semiconductors for Circularly Polarized Light Detection

ABSTRACT Rashba–Dresselhaus (RD) spin splitting arising from spin‐orbit coupling in spatial inversion asymmetric semiconductors is critical for realizing numerous advanced spintronic applications. However, modulating the RD spin splitting coefficient ( α RD ) is challenging due to the unclear structural‐property relationship. Herein, inspired by highly distorted inorganic selenites containing stereochemically active lone pair (SCALP) electrons, we selected 4‐aminomorpholine cation (4AM) containing SCALP electrons to synthesize a new two‐dimensional (2D) ferroelectric semiconductor (4AM) 2 PbBr 4 , exhibiting a giant α RD of 2.377 eV Å with Δ E RD = 126 meV and Δ k 0 = 0.106 Å −1 that differentiates circularly polarized light‐excited carriers in the momentum space via spin‐dependent optical transition selection rules with an asymmetric factor of 0.65. A geometric structure‐property relationship study reveals that incorporating SCALP into the organic cations enhances their net polarity and induces substantial distortions within the inorganic sublattices that enhance the α RD . This work reveals the regulatory mechanism of organic cations on distortion of inorganic octahedron and provides a new approach for efficient synthesis and rapid screening of 2D ferroelectric semiconductors with large α RD .